The temperature and colour of the EAGB stars in a galaxy are determined by
the age and metallicity of the population they represent. However there
remain a number of uncertainties in the comparison between the models and
the data (e.g.
Gallart et al. 1994;
Lynds et al. 1998).
It is
very important that more work is done to calibrate these very bright
indicators of past star formation events. In
Figure 3 theoretical EAGB
isochrones (from
Bertelli et al. 1992)
are overlaid on the HST CMD of the
post-starburst BCD galaxy, VII Zw403, and we can see that a large
population of EAGB stars is a bright indicator of a past high sfr,
and the luminosity spread depends upon metallicity and the age of the
sfr.

Figure 3. EAGB isochrones for
metallicities, Z = 0.001 and
Z = 0.004, are shown superposed on the observed CMD of
VII Zw403.
For each metallicity the isochrones are for populations of ages 1.3, 2, 3,
and 5 Gyrs, with the youngest isochrone being the brightest. This shows
the potential discriminant between the age and metallicity of older
populations, if the models could be calibrated.